The last three years of drought were the most severe that California has experienced in at least 1,200 years, according to a new scientific study published Thursday.

The study provides the state with breathtaking new historical context for its low reservoirs and sinking water tables, even as California celebrated its first good soaking of the season.

Analyzing tree rings that date back to 800 A.D. — a time when Vikings were marauding Europe and the Chinese were inventing gunpowder — there is no three-year period when California’s rainfall has been as low and its temperatures as hot as they have been from 2012 to 2014, the researchers found.

“We were really surprised. We didn’t expect this,” said one of the study’s authors, Daniel Griffin, an assistant professor in the University of Minnesota’s department of geography, environment and society.

The report, published in the journal of the American Geophysical Union, was written by researchers at Massachusetts’ Woods Hole Oceanographic Institution and the University of Minnesota.

The scientists measured tree rings from 278 blue oaks in central and southern California. Tree rings show the age of trees, and their width shows how wet each year was because trees grow more during wet years.

The researchers compared the information to a database of other tree ring records from longer-living trees like giant sequoias and bristlecone pines, dating back 1,200 years.

Meanwhile, the rain that California received this week provided a promising start to a winter that water managers say needs to be relentless and drenching to break the drought cycle.

‘Good beginning’

“It’s a good beginning,” said Art Hinojosa, chief of hydrology at the state Department of Water Resources. “But we need storm after storm after storm if we have any hope of getting out of the drought this year.”

By April, he said, California needs at least eight more major storm systems like the one this week — as well as many smaller storms — to fill its dangerously low reservoirs and break the drought. Rain and snow this winter needs to be at least 150 percent of average for the reservoirs to fill, Hinojosa said.

Above normal

This week’s storm was the biggest to hit California in roughly two years. Many parts of the state received between 2 and 4 inches of rain, doubling or tripling their totals since July. Through Thursday night, San Jose received 3.79 inches, San Francisco 4.43 inches and Oakland 3.01 inches, bringing each city’s rainfall to above-normal levels for the first time this year.